Marv made up for all those times he just had Garth show up to bring something to the surface for the NTT, with his usual "I have to get back to Atlantis" or "I'm not much good to you out of the water...blah blah".

Yeah, there were a lot of excuses as to why Garth wasn't able to hang around (my favorite being "Arthur's off chasing down Mera again, so I have to mind the baby" and that he liked being called "Uncle Aqualad"). Even without his magical abilities, there was a lot of value on land in having a kid around who could lift the weight of a truck over his head or flatten a dozen hired henchmen.